Various kinds of display devices, such as a home TV set, have been used as AV equipment or OA equipment. Examples of such display devices are a CRT (Cathode Ray Tube), a liquid crystal display device, a plasma display panel, an EL (Electroluminescent) display device, an LED (Light emitting Diode) display device, etc.
Recently, there has been an increasing demand for lighter, thinner, less power-consuming display devices with higher definition and a larger screen. Accordingly, display devices have been developed to meet the demand and some of them have been already put into practical use.
Of all the above display devices, the liquid crystal display device is particularly advantageous over the others. To be more specific, the liquid crystal display device has advantages that it has an extraordinary thin thickness (depth), consumes less power, and can display a full-color image. Because of these advantages, the liquid crystal display device is used in a wide variety of fields. Also, the liquid crystal display is expected to realize a display device with a large screen.
However, inconveniences, such as signal line disconnection and pixel defects, occur during the manufacturing process of the liquid crystal display device. The occurrence frequency of defective products is particularly high when manufacturing the liquid crystal display device with a large screen. As a necessary consequence, there is a problem that the price of the liquid crystal display device rises.
To solve the above problem, there has been proposed various types of liquid crystal display device adopting a multi-panel display method. More specifically, such a liquid crystal display device has a screen composed of a plurality of liquid crystal panels connected to one another. According to this arrangement, the screen of the liquid crystal display device can be enlarged.
However, according to the above arrangement, the joints connecting the liquid crystal panels, namely, the connection between the liquid crystal panels, become more noticeable, because light emanated from the back-light leaks through a space made in the joints. Therefore, to realize a liquid crystal display device capable of displaying a natural image on a large screen, a technique to make the joints unnoticeable must be developed.
Accordingly, the Applicant of the present invention proposes a new liquid crystal display device adopting the multi-panel display method in Japanese Laid-open Patent Application No. 122769/1996 (Tokukaihei 8-122769). FIG. 11 is a plan view schematically showing an arrangement of a liquid crystal display device 51 disclosed in the above publication. FIG. 12 is a cross section taken on line X--X in FIG. 11.
The liquid crystal display device 51 actually includes a plurality of active matrix type liquid crystal panels 52; however, assume that the liquid crystal 51 includes two liquid crystal panels 52 herein for the explanation's convenience.
Each liquid crystal panel 52 includes a TFT substrate and a CF substrate 54, both of which are transparent glass substrates. Although it is not illustrated in the drawing, a matrix of thin film transistors are placed on the TFT substrate 53, and a matrix of color filters 54a are placed on the CF substrate 54, whereby the liquid crystal display panel 52 is assembled as an active matrix type color liquid crystal panel. In addition, a black matrix 54b is placed on the CF substrate 54 to separate the pixels.
The TFT substrate 53 and CF substrate 54 are bonded to each other through a seal portion 55 which is provided along the circumferences of the TFT substrate 53 and CF substrate 54. A liquid crystal 56 is sealed airtight in a space between the TFT substrate 53 and CF substrate 54.
Both the liquid crystal panels 52 are laminated to a large reinforced substrate 57 with a bonding agent 58. Also, the liquid crystal panels 52 are connected to each other with the same bonding agent 58. In other words, the liquid crystal panels 52 are connected to each other adjacently on the same plane.
The bonding agent 58 is, for example, a UV-set bonding agent which is set when irradiated by UV rays. The bonding agent 58 has the same refractive index as the refractive index of the two glass substrate serving as the TFT substrate 53 and CF substrate 54 respectively. Thus, light passing through the joint is not refracted, reflected, nor scattered, thereby making the joint unnoticeable.
The outer surface of the reinforced substrate 57 (the side across where the liquid crystal 56 is sealed in) is covered with a polarizing plate (polarizer) 59 almost entirely. Likewise, the outer surfaces of both the liquid crystal panels 52 are covered with a polarizing plate (polarizer) 60 almost entirely. Here, the polarizing axes of the polarizing plates 59 and 60 intersect with each other at right angles.
Although it is not illustrated, a back-light composed of a cold cathode tube or the like is provided to the outer surface side of the polarizing plate 60, in other words, behind the liquid crystal display device 51 (lower half side in FIG. 12), and a driver for controlling an image signal is connected to each liquid crystal panel 52. Each liquid crystal panel 52 modulates light emanated from the back-light in accordance with image information to enable the observer to see the image information inputted therein.
Next, a manufacturing method of the liquid crystal display device 51 will be explained. FIGS. 13(a) through 13(d) are cross sections showing the manufacturing process of the liquid crystal display device 51.
As shown in FIG. 13(a), to begin with, a large quantity of the bonding agent 58 having a low viscosity of about 100-500 cP is applied to both the entire main surfaces of the liquid crystal panels 52 and a space between the liquid crystal panels 52. Then, as shown in FIG. 13(b), the reinforced substrate 57 is laminated to the liquid crystal panels 52. Here, the reinforced substrate 57 is laminated to the liquid crystal panels 52 from the edge gradually so as to be laminated firmly without trapping the bubbles under the bonding surface.
Then, as shown in FIG. 13(c), an excessive bonding agent 58a overflown from the spaces between the liquid crystal panels 52 and reinforced substrate 57 is removed. Then, as shown in FIG. 13(d), UV-rays are irradiated to the bonding agent 58 from the outer surface side of the reinforced substrate 57 to set the bonding agent 58.
However, the above prior art has four following problems.
1 The UV-set bonding agent 58 loses 5-10% of its volume when it is set. Thus, if the liquid crystal panels 52 are bonded to the reinforced substrate 57 through the bonding agent 58, the CF substrate 54 is pulled toward the reinforced substrate 57 side. Consequently, a cell gap of the liquid crystal panels 52 (thickness of the liquid crystal layer) varies, thereby making it impossible to obtain satisfactory display characteristics.
2 To laminate the liquid crystal panels 52 to the reinforced substrate 57 firmly and uniformly, the bonding agent 58 is overflown intendedly from the space between the liquid crystal panels 52 and reinforced substrate 57 when the liquid crystal panels 52 and reinforced substrate 57 are bonded to each other. Thus, although a large quantity of the bonding agent 58 is used, only 1/10 or less of which is set in practice, and the rest, that is, 9/10 or more, of the used bonding agent 58 is wasted or recycled. In other words, a considerable quantity of the bonding agent 58 is wasted, thereby increasing the manufacturing cost and hence the price of the liquid crystal display device 51.
3 When a pair of the liquid crystal panels 52 are bonded to each other in the manufacturing process of the liquid crystal display device 51, the excessive bonding agent 58a overflows to the periphery from the space between the liquid crystal panels 52 and reinforced substrate 57. Thus, besides a step of removing the overflown bonding agent 58a, the excessive bonding agent 58a adhering to the liquid crystal panels 52 and reinforced substrate 57 should be removed in another step. This keeps the workability quite low, and as a consequence, not only the throughput is reduced, but also the mass productivity remains quite low when the liquid crystal display device 51 is manufactured.
4 The liquid crystal panels 52, and the liquid crystal display panels 52 and reinforced substrate 57 are bonded to each other concurrently through the same bonding agent 58. Thus, if a defect, such as a crack, occurs at a portion in the joint of the liquid crystal panels 52, it is difficult to rework (remove) the defective portion.